Also new is the use of antibodies as drugs. These proteins, produced naturally by the immune system and manufactured in mammalian cell lines, can bind to specific molecules in the body and so block their action. They are being developed for use after transplant surgery to neutralise the action of substances that would otherwise trigger rejection, and for treatment of septic shock, cancer and other conditions.

Genetic engineering is also useful in fields other than medicine. Chymosin, the enzyme used to make cheese, can now be produced by yeast. Recombinant chymosin can then be used in the production of vegetarian cheese - in place of rennet extracted from a calls stomach.
Genetic engineering has also been used to create entirely new species of plants and animals. In plants, the new genes are introduced into cell cultures, which are then grown up by the methods of tissue culture described above. Many agriculturally important plant species, including the major food crops such as soya, rice, wheat and corn, have already been transformed.

The tomatoes stay firm longer and require less energy for processing. It has proved more difficult to create transgenic animals. Here, a fertilized egg is treated with the foreign DNA before it is transferred into the womb of a surrogate mother.

Such animals may be an improvement on the original - fish that grow much faster or animals with more lean than fatty meat, for example.

Researchers have even created pigs with 'humanised' organs to avoid rejection. Such xenografts could make up the shortfall in human donor organs. The animals bear a human protein called daf (decay accelerating factor) on their organs, which enables them to evade rejection by the human immune system.